Tensioners are widely used to maintain proper tension and prevent vibrations in power-transmitting chains. In a typical internal combustion engine timing drive, the tensioner is mounted on the engine, and applies pressure to a pivoted lever which is in sliding contact with the chain, thereby maintaining tension in the chain.
A hydraulic tensioner with a relief valve is described in United States Patent Application Publication No. 2002/0052259, published, May 2, 2002. The tensioner comprises a housing, and a hollow plunger, slidable in a hole in the housing, and protruding from the housing. A high pressure oil chamber is formed by the housing and the inside of the plunger. The plunger urged in the protruding direction by spring inside the high pressure oil chamber. Oil is introduced under pressure into the high pressure oil chamber through a check valve.
A relief valve is provided inside the plunger. The relief valve allows oil to flow out of the high pressure oil chamber through a discharge port in the protruding end of the plunger when the oil pressure in the high pressure oil chamber becomes excessive.
The relief valve comprises a relief valve seat press-fit into the hollow interior of the plunger, a valve body slidable into the valve seat, a biasing spring for biasing the valve body against the seat, and a separate, hollow, spring-receiving plug, also press-fit into the hollow interior of the plunger, for supporting the biasing spring. The relief valve seat includes a plurality of radial holes, and a plurality of axial grooves communicating with the radial holes. When the relief valve body moves away from its seat, oil in the high pressure oil chamber is allowed to flow outward through the radial holes, and axially, through the axial grooves, toward the spring-receiving plug.
One or more axial grooves are machined in the outer surface of the spring-receiving plug to allow oil, delivered through the relief valve, to flow past the plug toward the discharge opening at the protruding end of the plunger.
Because the spring-receiving plug and the relief valve seat are separately press-fit into the plunger, the axial grooves in the seat are not necessarily aligned with the axial grooves of the spring-receiving plug. Flow of oil through the discharge opening is dependent on the relative circumferential positions of the axial grooves in the seat and the plug. Thus, flow of oil can be impeded if the seat and plug are installed in such a way that the grooves are not in alignment, and performance of the relief valves can vary.
In addition, the depths of the axial groove or grooves formed on the outer surface of the spring-receiving plug are limited by the requirements for integrity and rigidity in the plug. Furthermore, the axial grooves in the plug are produced by machining, which requires a great deal of time and effort.